In a communications system where multiple channels are transmitted, such as a CATV system, multiple analog signals corresponding to the multiple channels may be combined into a wide-band multichannel RF signal, which drives a laser to produce a multichannel modulated optical signal. The multiple analog signals may include multiple modulated analog carriers that may be combined, for example, using frequency division multiplexing techniques. One or more digital signals modulated using digital modulation, such as quadrature amplitude modulated (QAM), may also be combined with the modulated analog carrier signals, for example, using subcarrier multiplexing (SCM) techniques. In some systems, for example, as many as 110 channels may be transmitted over a frequency range of about 50 MHz to 750 MHz.
Cross-modulation occurs when the nonlinearities of a system result in a carrier in a multi-carrier system (i.e., a multichannel RF signal) being modulated by the various signals carried on other channels in the same system. In a CATV system, for example, a group of video carriers may modulate other video carriers in a multichannel video system. Because each video channel contains a constant, high-level signal component at the horizontal line frequency (about 15.75 kHz in the NTSC system), this may be the most noticeable component of cross-modulation. One source of cross-modulation may be RF amplifiers in which gain compression produces less gain at higher RF signal power than at lower RF signal power. Another source of cross-modulation may be changes in bias current to reduce or prevent clipping in the laser, for example, as described in greater detail in U.S. patent application Ser. No. 12/053,104 filed Mar. 21, 2008, which is commonly owned and fully incorporated herein by reference.